Certain proposed Environmental Protection Agency regulations require that new gas-powered passenger vehicles be equipped with an on-board fuel vapor recovery system. The system will collect fuel vapors emitted from the gas tank as it is being refueled. A charcoal canister collects and contains these vapors. After the tank is filled with gasoline, and the engine is started, the accumulated vapors pass from the charcoal canister to the engine. Purging of the canister requires make-up air to fill the purged volume. This make-up air must be clean, fresh and dry. Any contaminants such as dirt, dust, or water entering the canister can affect the efficiency and operation of the canister.
The size of the charcoal canister is directly related to the fuel tank volume. A large canister volume requires mounting the canister on the underside of the car.